Optical fiber technology is currently utilized in communications systems to transfer information, e.g., voice signals and data signals, over long distances as optical signals. Over such long distances, however, the strength and quality of a transmitted optical signal diminishes. Accordingly, techniques have been developed to regenerate or amplify optical signals as they propagate along an optical fiber. When multiple different wavelengths are transmitted down the same optical fiber using wavelength division multiplexing (WDM), each of the wavelengths must be amplified. Amplification techniques for use in WDM systems have been implemented using Raman amplifiers and erbium doped fiber amplifiers (EDFAs).
Increasing the amount of information to be carried by telecommunications systems increases the demand for optical amplifiers having higher bandwidth to achieve the required amplification in a WDM system and particularly, an ultra-long haul dense wavelength division multiplexing (DWDM) system. Achieving gain equalization (i.e., flatness in amplification across the bandwidth) over a wider bandwidth has also been an objective of optical amplifiers. EDFAs have achieved gain equalization of the conventional-band or C-band over transoceanic distance. To greatly increase the capacity of ultra-long haul DWDM systems, however, gain equalization must be achieved beyond the EDFA's C-band. EDFAs have been able to achieve increased bandwidth by using the long wavelength band or L-band. However, C+L band EDFAs have had drawbacks such as complicated parallel optical designs and inferior noise performance compared to C-band EDFAs.
Hybrid Raman-EDFAs are known for their increased bandwidth and/or higher signal to noise ratio (SNR) compared to conventional C-band EDFAs. In a hybrid Raman-EDFA, Raman amplification can be used to assist the EDFA in the same bandwidth or to increase the bandwidth beyond the effective bandwidth of the EDFA. One way to increase the bandwidth is to use the EDFA as the C-band amplifier and use the distributed Raman effect to achieve the L-band amplification. The Raman gain and the EDFA gain dominate in the L-band and C-band, respectively, but also contribute to a certain extent in the adjacent band amplification. One example of a hybrid Raman-EDFA has provided continuous bandwidths over both the C-band and the L-band using erbium-doped fluoride fiber EDFAs and is described in greater detail by Hiroji Masuda, Shingo Kawai, Ken-Ichi Suzuki and Kazuo Aida in the article “75-nm 3-dB Gain-band Optical Amplification with Erbium-doped Fluoride Fibre Amplifiers and Distributed Raman Amplifiers in 9×2.5-Gb/s WDM Transmission Experiment,” Poc. of European Conf. On Optical Communications, ECOC '97, 15, 73–76 (1997).
These conventional hybrid Raman-EDFAs are designed for terrestrial systems. Because longer spans are typically used in such systems, the required gain is relatively large and the input power is relatively low. Thus, the terrestrial amplifier is optimally designed with multiple stages. Because the required gain is lower in a long-haul submarine system having shorter transmission spans, the multiple stage amplifier designs would actually degrade performance. Accordingly, there is a need for a hybrid Raman-EDFA capable of providing a substantially flat gain over a wider bandwidth in a submarine telecommunications system or other ultra-long haul telecommunications system.